Device to minimize the explosive effects of a metal casing in the event of accidental internal overpressure

ABSTRACT

To prevent the violent bursting of the casing of a thruster containing propergol during an accidental fire in its vicinity, a pyrotechnic composition ( 5, 6 ) is attached to this casing. This pyrotechnic composition locally heats the casing during this fire, and therefore embrittles this casing which opens at this place before the internal pressure becomes excessive. This prevents the fragmentation of the casing and the bursting out of the fragments.

BACKGROUND OF THE INVENTION

[0001] The present invention pertains to a device for minimizing theexplosive effects of a metal casing in the event of an accidental risein internal pressure.

[0002] Certain missile thrusters have a metal structure and reactviolently when subjected to heating because of surrounding fire. In thiscase, a large number of significantly sized splinters of the metalstructure are violently thrown out over long distances. This reactionresults from the combination of two phenomena:

[0003] whatever the architecture of the thruster, the heating due to thefire prompts a pyrotechnic reaction on the part of the thrusters. Theenergy thus released prompts a rise in pressure in the casing of thethruster;

[0004] simultaneously, the properties of mechanical resistance of thecasing are not sufficiently weakened by the fire. The casing breaks athigh pressure, giving rise to big metal splinters.

SUMMARY OF THE INVENTION

[0005] An object of the present invention is a device to minimize theexplosive effects of a metal casing subjected to internal overpressure,for example a missile thruster casing that gets excessively pressurizedif it is subjected to a fire, this device being easily adaptable todifferent sorts of casings while being inexpensive and efficient inalmost every instance.

[0006] The device according to the invention comprises a pyrotechnicsystem attached to the casing that is activated immediately upondetection of a phenomenon that causes a rise in overpressure in thecasing. The ignition of this pyrotechnic system gives rise, by heating,to a limited local degradation of the mechanical characteristics of thecasing.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] The present invention will be understood more clearly from thefollowing detailed description of an embodiment given by way of anon-restrictive example and illustrated by the appended drawing, whereinthe single FIGURE is a partial sectional view of a missile thrusterprovided with the device according to the invention.

MORE DETAILED DESCRIPTION

[0008] The present invention is described here below with reference to amissile thruster that undergoes an accidental fire in its storagehangar, but it is clearly not limited to this single application and canbe implemented in other applications comprising a metal casing, alreadypressurized internally, or else containing products that can place itunder normal pressure or overpressure in the event of accidents, thisoverpressure prompting the explosion of the casing with violentoutbursts of splinters from this casing.

[0009] The thruster 1 shown in the single FIGURE essentially has acircular cylindrical casing 2 enclosing a coaxial internal centralchannel 3, the space between this channel 3 and the casing 2 beingfilled with propergol 4. In order to provide for firm adhesion betweenthe propergol and the casing, the internal face of the casing is linedwith a binding agent 7 known as liner, generally made of polybutadieneor polyurethane.

[0010] According to the invention, a box 5 is attached to the externalface of the casing 2. This box 5 contains a pyrotechnic composition 6.This pyrotechnic composition 6, when fired, releases a large quantity ofcalories, sufficient to locally degrade the mechanical properties of thecasing to the point at which, when the propergol 4 undergoes spontaneousinflammation following its heating by a fire, the pressure thusgenerated in the casing does not have sufficient time to reach anexcessively high value and cause the above-mentioned violent reaction.In fact, as soon as this internal pressure starts rising, whileremaining relatively low (at a value that can be easily withstood by thecasing under normal conditions), the part of the casing embrittled bythe heavy heating due to the composition 6 opens, giving rise topractically no splinters at all, and lets out the pressurized gasesproduced by the inflammation of the propergol.

[0011] According to one embodiment of the invention, the composition 6is a propergol composition or a pyrotechnic composition, and its volumeis about 10 cm³ for a steel casing about 1.5 mm thick and 10 cm long.Naturally, the nature and quantity of the composition 6 are adapted tothe characteristics of the casing 2 and of the propergol 4.

[0012] The ignition of the composition 6 can be either spontaneousfollowing its heating due to the fire or activated by an appropriateignition device connected to a heat sensor attached to the casing 2.

[0013] According to another characteristic of the invention, a layer ofheat-insulating material is placed around the casing. Thisheat-insulating material is used to delay the heating of the propergol 4to allow time for the composition 6 to sufficiently degrade the localmechanical characteristics of the casing 2.

[0014] According to one variant of the invention, the box 5 with thecomposition 6 is attached to the interior of the casing 2, in contactwith this casing. An embodiment of this kind, although less simple tomake than embodiment described here above, may have advantages in otherapplications, for example owing to the fact that the box 5 is not an“outgrowth” one the casing and hence does not risk getting accidentallywrenched off, for example during the handling of the casing, or does notin any way constitute a hindrance, for example when the casing is beingshifted.

[0015] In general, the device of the invention prevents the violentexplosion of a casing which either contains products capable of catchingfire by spontaneous inflammation or exploding through an excessiveheating of the casing or else is under internal pressure in normalconditions whereas, however, this internal pressure may riseexaggeratedly under normal conditions (over-heating, modification of thecomposition of the contents, internal short circuit etc.). In suchcases, the ignition of the compound 6 is activated spontaneously or byan appropriate sensor of physical or chemical conditions (such as apressure sensor, temperature sensor, gas sensor etc.).

What is claimed is:
 1. A device for minimizing the explosive effects ofa metal casing subjected to internal overpressure, comprising apyrotechnic system attached to the casing that is activated immediatelyupon detection of a phenomenon that causes a rise in overpressure in thecasing, the ignition of this pyrotechnic system causing a limited localdegradation, through heating, of the mechanical characteristics of thecasing.
 2. A device according to claim 1, wherein the pyrotechnic systemis attached to the exterior of the casing, in contact with it.
 3. Adevice according to claim 1, wherein the pyrotechnic system is attachedto the interior of the casing, in contact with it.
 4. A device accordingto claim 1, 2 or 3, wherein the ignition of the pyrotechnic system isprompted by heating.
 5. A device according to one of the claims 1 to 3,wherein the ignition of the pyrotechnic system is activated by a sensorof physical or chemical conditions.
 6. A device according to one of theabove claims, wherein the casing is lined with a layer ofheat-insulating material.
 7. A device according to one of the aboveclaims, wherein the casing is that of a missile thruster.